The present invention generally relates to methods and systems for providing radiocommunications and, more particularly, to such methods and systems which use satellites to provide radiocommunications.
In the past, satellite systems for providing global coverage have been of one of three types, broadly classifiable by orbiting distance into geostationary (GEO), low earth orbit (LEO) and medium earth orbit (MEO). An example of a geostationary satellite communications system is the INMARSAT system (International Maritime Satellite Organization). One advantage of geostationary satellites is that they remain in a fixed position relative to the earth, and only four such satellites are required to illuminate the entire earth. A disadvantage of geostationary satellites is that they are very distant, needing high transmit power and large antennas to provide communications capacity and incurring about a xc2xc second, round-trip, signal propagation delay.
An example of a LEO system is the IRIDIUM system proposed by Motorola. An advantage of LEO systems is that the satellites are much closer to the earth, thereby providing improved communications. Since the satellites are closer to the earth, less transmitting power is needed for both the satellite and an individual user""s transceiver. A disadvantage is that about 70 satellites are required to give 24 hour coverage to most points on the globe. Moreover, satellites in low earth orbits move quite rapidly relative to the earth, thereby causing high Doppler shifts and frequent handovers of communication from one satellite to the next.
An example of the compromise MEO system is the ODYSSEY satellite system proposed by TRW. The orbital altitude of MEO satellites lies between the GEO and LEO orbits, providing better communication quality than a GEO system, with less movement and Doppler shift than an LEO system. Moreover, MEO systems provide more or less 24 hour coverage to most points on the globe using between 8 and 18 satellites which is much less expensive than the about 70 satellite LEO solution.
While the MEO solution represents a good compromise between conflicting requirements, it suffers from a practical disadvantage that almost all satellites must be in place before coverage is sufficient (in percentage of time available) to be considered attractive to subscribers. This lesson was learned from the GPS satellite navigation system, which is also a MEO solution. Thus, a considerable investment spanning a multi-year program is needed before significant revenue can be expected when implementing a MEO system.
Accordingly, it would be desirable to provide radiocommunication systems and methods which overcome the foregoing drawbacks of conventional LEO, MEO and GEO solutions.
According to exemplary embodiments of the present invention a hybrid GEO/MEO solution begins life with the launch of a geostationary satellite that provides radiocommunication coverage to a region of major expected traffic growth, but has a limited capacity which is sufficient to support only an initial number of subscribers. This is followed by the successive launch of a number of MEO satellites. The MEO satellites can, initially, supplement the coverage of the geostationary satellite. Later, once sufficient MEO satellites are in orbit, the primary traffic burden can be relegated to the MEO satellites, with the GEO satellite performing a supplementary role. Finally, if desired, enough MEO satellites can be launched to provide all of the desired system capacity.
In this way, a major drawback of MEO systems, specifically the lengthy period between initial launching and sufficient capacity to reach profitability, is overcome since systems according to the present invention provide instant capacity by first launching a geostationary satellite.